Ni Alloys

Question 1

What properties are required for a material to operate at a high temp?

Answer 1

• High melting point
• Withstand a load close to its melting temp
• Resistance to creep deformation
• Microstructural stability

Question 2

What are the advantages of using FCC materials for severe temp uses?

Answer 2

BCC often become brittle at low temps etc.

Whereas FCC are better, and as they’re close-packed, diffusion is easier. (Nickel is FCC)

Question 3

What are some of the physical properties of Nickel?

Answer 3

Heavy, high melting point, exhibits no allotropy so is FCC up to melting point and therefore no volume changes.

Question 4

What is the microstructure of a nickel alloy?

Answer 4

FCC gamma solid solution with gamma prime precipitates (up to 0.7 volume fraction)

Gamma prime precipitates are cuboidal shaped.

Question 5

What does an ordered atom mean?

Answer 5

The Nickel atoms are always at the face centres and additions are always at corners.

Question 6

What does a coherent precipitate mean?

Answer 6

Precipitate fits well into the matrix - similar lattice parameter which reduces the boundary energy between matrix and prec. and makes it a more stable structure.

Question 7

Why was there a move from forging to casting?

Answer 7

Alloys became too difficult, casting allows more complex shapes like hollow blades for better cooling.

Question 8

What are single crystal blades and why and how do we produce them?

Answer 8

They are produced from a single crystal.

Grain boundaries are removed entirely by a ‘grain selector’ to the base of the wax mould - only a single grain enters the casting cavity.

Question 9

What could the presence of carbon do in a Ni based super alloy?

Answer 9

Can lead to cracking of boundary particles and their decohesion from grain boundaries.

Can also lead to low melting point phases and cause incipient melting which limits its heat treatment temps.